The invention relates to a method for producing an exposed substrate, which has at least two image areas.
For protecting security documents, such as bank notes, ID cards or the like, optically variable elements are often used, which are made up of diffraction gratings. In the following such elements are referred to as grating images. These may be grating images, in which for the viewing the first and higher diffraction order is made use of, as it is the case with e.g. holograms or with grating images that are composed of grating areas. Alternatively, grating images are used, in which the zero diffraction order is made use of, as described for example in U.S. Pat. Nos. 4,892,385 and 4,484,797.
The first-order grating images and the zero-order grating images mainly differ from each other in that with the first-mentioned the grating constant has to be greater than the light wavelength, while with the last-mentioned the grating constant preferably is selected smaller than the wavelength, in particular when one wants to observe the pure zero order. While for first-order grating images the grating constant is decisive for the color variability and the grating line structure plays an only minor role, with the zero-order grating images it is precisely the other way round.
The diffraction structures used as security elements mostly are produced as embossed holograms. For that purpose a photoresist layer applied to a substrate is exposed with laser light or with electron beams. The term photoresist refers to film-forming materials sensitive to radiation, e.g. photoresins, the solubility behavior of which changes when exposed to light or radiation. One differentiates between positive- and negative-working photoresists. The first-mentioned become easily soluble by photochemical degradation or conversion of active groups when exposed to radiation, while the last-mentioned become hardly soluble or insoluble by crosslinking or photopolymerization.
The development of the photoresist layer leads to a peak-and-valley structure, which can be galvanically molded. In first-order grating images the profile structure preferably has a sinusoidal form, in zero-order grating images it has a box-type or trapezoidal form. The molded structure can be duplicated and used for producing embossing dies.
Furthermore, grating images are known for which a plurality of exposure steps have to be combined with each other. For this purpose mainly two methods are known.
In a first method partial areas of a photoresist layer are covered with the help of masks and then the unmasked partial areas of the photoresist layer are exposed with e.g. laser light of a first wavelength as to produce a diffraction structure. In further procedure steps the already exposed parts of the photoresist layer are covered and the parts of the image now freed of the masks are exposed with e.g. laser light of different wave lengths as to produce further diffraction structures.
This method has the disadvantage that it cannot be used, if in a grating image different resist layer thicknesses are required, as it is the case e.g. when zero-order grating images are combined.
In a further known method this problem is avoided by galvanically producing a plurality of embossing dies out of independently exposed photoresist layers. Each embossing die contains only a partial area of the complete image. In order to obtain the complete image the embossing dies are embossed side-by-side into thermoplastic material. With this method, however, it is disturbing that such a side-by-side embossing leads to the forming of seams.
Starting out from this prior art it is the problem of the invention to provide a method, wherein the exposure with different types of radiation can be effected in a simple fashion and wherein, optionally, the layer thickness in different areas can be adjusted to the exposure.